U.S. patent number 6,520,646 [Application Number 09/746,808] was granted by the patent office on 2003-02-18 for integrated front projection system with distortion correction and associated method.
This patent grant is currently assigned to 3M Innovative Properties Company. Invention is credited to Patricia H. DeLuca, Ernesto M. Rodriguez, Jr., Steven G. Saxe.
United States Patent |
6,520,646 |
Rodriguez, Jr. , et
al. |
February 18, 2003 |
**Please see images for:
( Certificate of Correction ) ** |
Integrated front projection system with distortion correction and
associated method
Abstract
A front projection system and associated method are disclosed
that correct distortion for a front projector device. The front
projector device is characterized by off-axis optics and a
throw-to-screen diagonal ratio of at most 1 which generate
distortion including pincushion, keystone and anamorphic distortion
components. Further, the projection device has an electronic
distortion correction component operable to pre-distort an image
prior to projection in order to correct for the pincushion,
keystone and anamorphic distortion components of the projector
device. Further, an associated method is also disclosed for
correcting distortion generated in a projection system by a
combination of optical and electronic correction. The method
comprises setting a limit on the amount of image information that
is acceptable to lose through the optical components, and selecting
an optical solution is comprising optics having inherent distortion
within the set limit. Then, an electronic correction component is
selected that is operable to pre-distort an image to correct for
remaining distortion not corrected by the optics of the optical
solution.
Inventors: |
Rodriguez, Jr.; Ernesto M.
(Austin, TX), Saxe; Steven G. (Round Rock, TX), DeLuca;
Patricia H. (Round Rock, TX) |
Assignee: |
3M Innovative Properties
Company (St. Paul, MN)
|
Family
ID: |
27087799 |
Appl.
No.: |
09/746,808 |
Filed: |
December 22, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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616563 |
Jul 14, 2000 |
6394609 |
|
|
|
261715 |
Mar 3, 1999 |
6179426 |
|
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Current U.S.
Class: |
353/69;
348/E5.137; 353/70 |
Current CPC
Class: |
H04N
5/74 (20130101); H04N 9/3185 (20130101); H04N
9/3194 (20130101); G03B 21/10 (20130101); G03B
21/145 (20130101) |
Current International
Class: |
G03B
21/14 (20060101); H04N 5/74 (20060101); G03B
021/14 () |
Field of
Search: |
;353/69,70,79,122
;345/647,649 |
References Cited
[Referenced By]
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Foreign Patent Documents
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0 773 678 |
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May 1997 |
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EP |
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0 825 480 |
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Feb 1998 |
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EP |
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0 837 351 |
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Apr 1998 |
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Dec 1998 |
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1 018 842 |
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Jul 2000 |
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EP |
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05297465 |
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Nov 1993 |
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JP |
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11331737 |
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Nov 1999 |
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JP |
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2000206452 |
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Jul 2000 |
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JP |
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WO 00/33564 |
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Jun 2000 |
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WO |
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01 43961 |
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Jun 2001 |
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WO |
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Other References
International Search Report PCT/US02/03747 dated Jul. 12,
2002..
|
Primary Examiner: Dowling; William
Attorney, Agent or Firm: Florczak; Yen Tong Ho; Nestor
F.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent
application Ser. No. 09/261,715, filed on Mar. 3, 1999 and entitled
"Integrated Projection System", and now U.S. Pat. No. 6,179,426,
and of U.S. patent application Ser. No. 09/616,563, filed Jul. 14,
2000, now U.S. Pat. No. 6,394,609, both which are hereby
incorporated by reference.
Claims
What is claimed is:
1. A method for manufacturing a projection system projecting a
distortion corrected image, the projection system including optical
projection components and an electronic imaging component, the
method comprising the steps of: a. determining the acceptable data
loss, DL.sub.ACC for the projection system, b. determining the
optical projection parameters of the projection system, including
projection angle and throw ratio; c. calculating distortion
components for a not-electronically corrected image under the
optical projection parameters; d. determining the shape of the
not-electronically corrected image when projected; e. calculating a
warp map for the correction of the distortion components; f.
applying the warp map to the electronic imaging component; g.
measuring the electronic data loss, DL.sub.E, in the electronic
imaging component; h. if DL.sub.E >DL.sub.ACC, then provide an
optical correction mechanism that reduces the distortion
components; and i. repeat steps c through h until
DL.sub.E.ltoreq.DL.sub.ACC.
2. The method of claim 1, further including the step of evaluating
whether the system constraints of the optical design are met and if
not, redesigning the optical design.
3. The method of claim 2, wherein the system constraints include
cost.
4. The method of claim 1, wherein the step of calculating
distortion components includes calculating pincushion, keystone,
and anamorphic distortion components.
5. The method of claim 1, wherein the step of determining the
acceptable data loss includes evaluating the screen image
resolution desired for the particular application of the projection
system.
6. The method of claim 1, wherein the projection system is an
off-axis, small throw ratio front projection system.
7. The method of claim 6, wherein the projection system includes an
integrated screen design having a constant throw ratio, throw
distance and offset angle.
8. The method of claim 1, wherein the electronic imaging component
is an XGA resolution imager and the acceptable data loss is
determined to be 40%.
9. A projection system manufactured in accordance with the method
of claim 1.
10. A front projection system, comprising: a. a front projector
device including: i. off-axis optics and a throw ratio of at most
1, which generate lens and geometric distortion components; ii.
optical distortion correction; and iii. an electronic distortion
correction operable to pre-distort an image prior to projection by
the projection device; iv. wherein the combined optical and
electronic correction are selected to essentially correct for both
the lens and geometric distortion components of the projector
device, while maintaining data loss below a minimum acceptable
level.
11. The front projection system of claim 10, wherein the lens
distortion component comprises pincushion components.
12. The front projection system of claim 10, wherein the geometric
distortion component comprises anamorphic and keystone distortion
components.
13. The front projection system of claim 10, further including an
integrated screen, wherein the front projector device is integrally
coupled to a projection screen having a constant throw ratio and
offset angle.
14. The front projection system of claim 13, wherein the front
projector device comprises an arm movably coupled to the projection
screen.
15. The front projection system of claim 10, wherein the electronic
distortion correction component comprises an integrated circuit
chip having image predistortion functions.
16. A method for correcting distortion generated in a projection
system, comprising: a. providing an off-axis projection system
having a throw-to-screen diagonal ratio of at most 1; b. setting a
limit on the amount of image information that is acceptable to lose
through the optical components; c. selecting an optical solution
comprising optics having inherent distortion within the set limit;
and d. selecting an electronic correction component operable to
pre-distort an image to correct for remaining distortion not
corrected by the optics of the optical solution.
17. The method of claim 16, wherein an XGA resolution imager is
used to create the image and the step of setting the limit
comprises setting a limit of approximately 40% loss of image
information.
18. The method of claim 16, wherein selecting an optical solution
comprises selecting a lens having a 9.44 mm focal length.
19. The method of claim 16, wherein selecting an electronic
correction component comprises selecting an integrated circuit chip
having image pre-distortion functions.
20. The method of claim 16, wherein selecting an optical solution
comprises selecting a lens having a 9.44 mm focal length that
generates approximately 10% pincushion distortion, approximately
74% keystone distortion and approximately 34% anamorphic
distortion.
21. The method of claim 16, wherein the providing step further
comprises providing a front projector device within the projection
system that is movably coupled to a projection screen.
22. The method of claim 16, wherein the step of selecting an
electronic correction component comprises utilizing an integrated
circuit chip having image pre-distortion functions.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for
displaying an optically corrected image using a small throw ratio,
off axis projection display system. In a is particular embodiment,
the present invention relates to an off-axis integrated front
projection system having a throw ratio less or equal to 1.0 that
coordinates specialized projection optics and electronics optimized
to work together to create a high-quality viewing image and correct
lens induced optical distortion, keystone distortion, and
anamorphic distortion.
Electronic display systems are devices capable of presenting
electronically generated images. Whether for use in
home-entertainment, advertising, videoconferencing, computing,
data-conferencing or group presentations, the demand exists for an
appropriate display device.
Image quality remains a very important factor in choosing a video
display device. However, as the need increases for display devices
offering a larger picture, factors such as cost and device size and
weight become vital considerations. Larger display systems are
preferable for group or interactive presentations. The size of the
display system cabinet has proven an important factor, particularly
for home or office use, where space to place a large housing or
cabinet may not be available.
Currently, the most common video display device is the typical CRT
monitor, usually recognized as a television screen. CRT devices are
relatively inexpensive for applications requiring small to medium
size images (e.g., 9" to 27", .about.23 to 70 cms). (image size
traditionally is measured along the diagonal dimension of a
rectangular screen). However, as image size increases, the massive
proportions and weight of large CRT monitors become cumbersome and
severely restrict the use and placement of the monitors. Also,
screen curvature issues appear as the screen size increases. Large
CRT monitors consume a substantial amount of electrical power and
produce significant electromagnetic radiation. Finally, the cost of
very large CRT monitors may be prohibitive for many
applications.
A new category of presentation systems includes so-called thin
plasma displays. Much attention has been given to the ability of
plasma displays to provide a relatively thin (about 75-100 mm)
cabinet, which may be placed on a wall as a picture display in an
integrated compact package. However, at the present time, plasma
displays are costly and suffer from the disadvantages of low
brightness (approx. 200-400 cd/m.sup.2 range) and difficulty in
making repairs. Plasma display panels are heavy (.about.80-100
lbs., 36-45 kg.), and walls on which they are placed may require
structural strengthening.
A traditional type of video presentation device is the projection
system, including both rear and front projection. In projection
systems, one or more imagers creates an image that is projected
using optical lenses. An imager generally is an electronically
controlled array of pixels that can be turned on or off to create
an image. Imagers, or light valves as they are sometimes called,
may be reflective (an "on" pixel reflects incident light to form
the image) or transmissive (an "on" pixel transmits incident
light). Common imager types include liquid crystal display devices
and digital micromirror devices.
Rear projection generally comprises a projection mechanism or
engine contained within a large housing for projection to the rear
of a transmissive screen. Back-projection screens are designed so
that the projection mechanism and the viewer are on opposite sides
of the screen. The screen has light transmitting properties to
direct the transmitted image to the viewer.
A front-projection system is one that has the projection mechanism
and the viewer on the same side of the screen. Front projection
systems present many different optical and arrangement challenges
not present in rear projection systems, as the image is reflected
back to the audience, rather than transmitted. An example of a
front projection system is a portable front projector and a front
projection screen, for use in meeting room settings or in locations
such as an airplane cabin.
Front projection systems have traditionally not been considered
attractive for interactive applications because of factors such as
blocking of the image by the projector or the presenter and image
distortion.
Traditional electronic front projectors typically require a room
that affords the projection volume necessary for image expansion
without physical obstructions. Although images may be projected
upon a large flat surface, such as a wall, better image quality is
achieved by the use of a separate screen. FIGS. 1 and 2 illustrate
a traditional front projection system. A projector 10 is placed on
a table or other elevated surface to project an image upon a screen
or projection surface 20.
Traditional integrated projectors require optical adjustment, such
as focusing every time the projector is repositioned, as well as
mechanical adjustment, such as raising of front support feet to
position the image on the projection screen. Electronic
connections, such as those to a laptop computer, generally are made
directly to the projector, thus necessitating that the projector be
readily accessible to the presenter or that the presenter runs the
necessary wiring in advance.
To achieve a suitable image size, and also due to focus
limitations, the projector 10 requires a certain "projection zone"
and distance from the screen 20. Table A lists the published
specifications for some common electronic projectors currently in
the market.
TABLE A Smallest Shortest Maximum Projector Lens Focal Imager
Screen Throw Throw Keystone Type Length Diagonal Diagonal Distance
Ratio Correction CTX Opto * 163 mm 1.0 m 1.1 m 1.1 20.degree.
offset/ ExPro 580 Transmissive optical LCD InFocus * 18 mm 1.3 m
1.5 m 1.2 18.degree. offset LP425 Reflective DMD Chisholm 43-58.5
mm 23 mm 0.55 m 1.2 m 2.2 15.degree. electronic Dakota Reflective
X800 LCD Epson 55-72 mm 33.5 Transmis- 0.58 m 1.1 m 1.9 * Powerlite
sive LCD 7300 Proxima 45-59 mm 23 mm 0.5 m 1.0 m 2.0 12.degree.
offset Impression Transmissive A2 LCD 3M 167 mm 163 mm 1.0 m 1.2 m
1.2 16.degree. offset/ MP8620 Transmissive optical LCD * Not given
in published specifications
Throw distance is defined as the distance from the projection lens
to the projection screen measured along the optical axis of the
projection lens. Throw ratio usually is defined as the ratio of
throw distance to screen diagonal. Short throw distance is defined
as at most one meter. To achieve a large image, between .about.40
to .about.60 inches (.about.1 to .about.1.5 meters), a projector
having a throw ratio of approximately 1.5 must be positioned at
least 3.5 to 7.5 feet (approximately .about.1.5 to .about.2.25
meters) away from the wall or screen.
The existence of this "projection zone" in front of the screen
prevents the viewer from interacting closely with the projected
image. If the presenter, for example, wishes to approach the image,
the presenter will block the projection and cast a shadow on the
screen.
The projection zone may be reduced by moving the projector closer
or off-axis from the screen. However, optical distortion effects
significantly affect the quality of a projected image at short
throw, small throw ratio, and offset angles. There are three
distortion effects of particular concern in off-axis projection,
especially in front projection systems: keystone geometric
distortion, anamorphic geometric distortion, and projection lens
imaging distortions, such as third order pincushion distortion. The
effects of these distortion components are increased, the closer
the projection lens is to the screen.
Those familiar with the use of electronic projectors will
appreciate that placing the projector at an angle to the central
normal axis of the screen (i.e., off-axis) produces a trapezoidal
shape distortion of the image, known as a keystone effect.
Keystoning is a geometric image distortion where the projection of
a rectangular or square image results in a screen image that
resembles a keystone or trapezoid, that is a quadrilateral having
parallel upper and lower sides, but said sides being of different
lengths.
Methods for the reduction of keystoning again are dependent upon
the position of the projector with respect to the screen. A measure
of keystone correction may be achieved by optical and by electronic
methods. For moderate (10.degree.-20.degree. off axis) keystone
correction in LCD projectors, optical methods are presently
preferable, as electronic methods may suffer from pixelation, as
pixels become misaligned with the image features. Most new
electronic projectors offer a limited degree of optical keystone
correction, often achieved by mechanical offset of the projection
components. However, the placement of the projector at moderate
offset angles may still interfere with the line of sight of the
audience.
Anamorphic distortion causes a projected image to be stretched
unequally in the vertical direction above and below the optical
axis centerline.
Pincushion distortion is a third order distortion generated by the
projection lens. The degree of pincushion distortion is related to
the complexity of the lens and to the lens design.
Available optical keystone correction in presently commercially
available large throw ratio and throw distance portable electronic
front projectors generally ranges between 10.degree. to 20.degree..
While it may be theoretically possible to correct these distortion
components in small throw-ratio, off-axis systems by optical means,
the cost of developing and manufacturing specialized exotic lenses
may be so high as to destroy the commercial attractiveness of such
a system. Similarly, electronic means of image distortion
correction are deficient at short throw (less than 1 meter), small
throw ratio (less or equal to 1), off axis projection, due to the
effects of image pixelation and misalignment.
Image pixelation occurs when the pixels in a projection imager
become misaligned with the image features. No commercially
available correction mechanism has addressed satisfactorily the
issues of optical distortion and image quality in small
throw-ratio, short throw, off-axis front projection systems.
A newer means for electronic image correction is discussed in U.S.
Pat. No. 5,594,676, issued to Greggain, et al., entitled "Digital
Image Warping System" and in J. Goel, et al., "Correcting
Distortions in Digital Displays and Projectors Using Real-Time
Digital Image Warping," SID 99 Digest, pp. 238-241 (hereinafter
"SID Article"). Under this system of electronic correction an image
handling microchip predistorts the output image on an imager (such
as an LCD or DMD imager) to compensate for distortions introduced
by the rest of the system. This system corrects over a wider range
and corrects more types of distortion.
However, such system requires extensive computing power and a
dedicated image handling IC. Even more importantly, the
pre-distortion of the image causes significant loss of data. For
example, an image in an XGA imager contains 1024.times.768 pixels
of image data. If the image is pre-distorted as discussed in the
SID Article, only a fraction of those pixels may be used. The
imager bounds the entire pre-distorted image. Therefore, in the
best case scenario, if the pre-distorted image may be correctly
formatted to use the maximum possible area in the imager, only the
widest point of the pre-distorted image would use all of the pixels
in a row. Similarly, only the tallest portion of the pre-distorted
image would use all of the pixels in a column. For all other rows
and columns, pixel data necessarily will have to be omitted. As the
amount of distortion increases, the shape variations of the
pre-distorted image increase. Therefore, the more severe the
correction necessary, the more information will be lost. While such
information loss may not be critical in some applications, the data
loss may severely affect certain applications, such as static
presentations including small features or numbers.
The need remains for a large screen video presentation system that
offers efficient space utilization, and allows for user
interaction. Such a system should preferably correct the various
distortion components within a displayed image while minimizing
data loss.
SUMMARY OF THE INVENTION
The present invention provides a method for manufacture for an
off-axis, small throw ratio projection system. The system of the
present invention greatly reduces the projection zone by placing
the projector at a short distance and off-axis from the projection
screen. A reduced and displaced projection zone offers immediate
and attractive advantages. A large image achieved by a projector
closer to the screen allows the presenter to get closer to the
screen without interfering with the image and reduces the space
required for front projection.
However, small throw ratio (where the throw ratio is less equal to
1), off-axis projection induces large distortion effects. The
present invention addresses these distortion effects by a novel
method of combining optical correction with electronic
correction.
The present invention provides a projection system and associated
method that corrects, that is reduces or eliminates, distortion
components within a projected image. In one embodiment, the present
invention includes a front projection system having a front
projector device coupled to a projection screen. The front
projector device is characterized by off-axis projection and a
throw ratio of at most 1, which generate distortion including
pincushion, keystone, and anamorphic distortion components. The
projection device has an electronic distortion correction component
operable to pre-distort an image prior to projection. The method of
the present invention combines optical and electronic correction in
order to correct for the pincushion, keystone and anamorphic
distortion components of the projector device.
In one embodiment, the optics includes a 9.44 mm focal length
wide-angle projection lens that generates approximately 10%
pincushion distortion. The electronic distortion correction
component includes an integrated circuit chip having image
pre-distortion functions. Off-axis optics further generate
approximately 74% keystone distortion and approximately 34%
anamorphic distortion. According to another aspect of the present
invention, a method is disclosed for correcting distortion
generated in a projection system by a combination of optical and
electronic correction. The method includes setting a limit on the
amount of image information that is acceptable to lose through the
optical components, and selecting an optical solution, including
optics having inherent distortion within the set limit. An
electronic correction component is then selected that is operable
to pre-distort an image to correct for remaining distortion not
corrected by the optics of the optical solution. In a particular
embodiment, the projection lenses are placed at a predetermined
distance and angle from the projection screen. Electronic
correction is achieved by "warping" the electronic image in
accordance with a warp map that accounts for the particular
distortion components at the specific throw distance and projection
angle.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a traditional projection device and
screen arrangement.
FIG. 2 is an elevation side view of the arrangement illustrated in
FIG. 1.
FIG. 3 is a perspective view of an integrated front projection
system in accordance with the present invention in the use or
projection position.
FIG. 4 is a perspective view of the integrated front projection
system illustrated in FIG. 3 in the closed or storage position.
FIG. 5 is a top plan view of the integrated front projection system
illustrated in FIG. 3.
FIG. 6 is a side elevation view of the integrated front projection
system illustrated in FIG. 3 in the use or projection position.
FIGS. 7A and 7B are a cross sectional schematic side elevation view
and a crosssectional schematic plan view of the projection head and
arm of the front projection system illustrated in FIG. 8.
FIG. 8 is a perspective view of a second embodiment of a front
projection system in accordance with the present invention.
FIG. 9 is a side elevation of a third embodiment of a projector
system in accordance with the present invention.
FIG. 10 is a plan view illustrating distortion components that can
occur in an uncorrected short-throw, off-axis projection system
like the present integrated front projection system.
FIGS. 11A and 11B are cross-sectional views of example optical
systems characterized by being on-axis, and off-axis.
FIG. 12 is a plan view of tilted projection on a vertical
screen.
FIG. 13 is a flow diagram view of one embodiment of the method of
the present invention.
FIG. 14 is a flow diagram of a second embodiment of the method of
the present invention.
FIG. 15 is a graphical conceptual view of a method for correcting
distortion generated in a projection system in accordance with the
present invention.
FIG. 16 is an outline of the shaped image.
FIGS. 17A and 17B are line diagrams of the projection ray path.
FIG. 18 is a side elevation schematic of a system including both
optical and electronic correction.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 3-7 illustrate a first exemplary embodiment of an integrated
front projection system 100 in accordance with the present
invention. A particular embodiment of the present invention
includes a small throw ratio, off-axis front projection system that
integrates an optical engine, having modular control and power
supply electronics, and a dedicated projection screen to provide a
compact video display device. The term "small throw ratio" is
defined as having a throw ratio .ltoreq.1.
The front projection system 100 includes a body 101, and a
dedicated high gain projection screen 102 mounted on a frame 104. A
projection head 106 is pivotally mounted by an arm 108 to a center
top portion of the frame 104 at a hinge unit 110.
At the top and center of the frame 104, the hinge unit 110 allows
the projection arm 108 and head 106 to pivot between a closed
(storage) position and an open (use) position. FIG. 4 illustrates
the projection system 100 in a closed or storage position. When not
in use, the arm 108 may be kept in the closed position as to be
substantially parallel with the frame 104, and thus present no
obstruction to objects that may be moving in the space in front of
the frame 104. Although the arm 108 is shown folded back to an
audience left position, the system may be adaptable to allow
storage of the arm and projection head to an audience right
position. An ability to select storage position may be valuable in
avoiding obstacles present in the projection area prior to the
installation of the system. The ability of the arm 108 to rotate
contributes to the projection system's minimal thickness.
As illustrated in FIG. 5, the arm 108 may be rotated a
.+-.90.degree. for storage on the right or the left side. In
alternative embodiments, the arm may rotate less or more than
90.degree. to place the head in a predetermined position.
In the present embodiment, the screen 102 is optically coupled to
the projection head. The screen 102 may be a flexible material
extended over frame 104 or may be a rigid component. In an
alternative embodiment, both the screen and the frame are made of
an integral sheet of material, that may be retractable into the
main body 101. The screen 102 may include multiple-layers or
special coatings, such as to allow its use as an erasable
whiteboard. Alternative embodiments of the screen may comprise 3M
multi-layer film technology, for example, as described in U.S. Pat.
No. 6,018,419, assigned to 3M.
The frame 104 contains and supports other components of the system.
The frame 104 may house additional components such as integrated
speakers 112, input and output jacks 113, and a control panel 114.
In the present exemplary embodiment, the mechanical infrastructure
of the projection system 100, the arm 108 and the frame 104,
include lightweight materials such as aluminum, magnesium, or
plastic composites.
Referring to FIG. 6, when active, the projection system 100
generates a beam of light 162 having a plurality of light rays. In
relation to a coordinate system wherein the screen defines a
z-plane, each light ray includes components along both the
horizontal x-plane and the vertical y-plane. The angle of incidence
of each light beam upon the screen 102 depends on the optical
characteristics of the projector, such as f-number, and the
position of the projection head 106 in relation to the screen
102.
As explained in more detail in relation to FIGS. 10-18, the system
100 optimizes the coupling of the projection engine with the exact
positioning of the head 106 in relation to the screen 102 to yield
high contrast, brightest enhancement, image uniformity, optimal
image position, and sharp focus. Since the optical parameters
(e.g., offset angle, throw ratio) of the projection system are
known and the exact position of the projector head 106 in the use
position is known and predetermined, the exemplary screen 102 may
be designed and optimized to provide maximum illumination for the
audience while reducing interference by ambient light.
As may be appreciated in FIG. 6, the projection system 100 places
the projection head 106 at an off-axis angle and close (short
throw) distance to the screen 102, thus minimizing the possibility
of the presenter's interference. Placement of the optical head 106
at such sharp off-axis angle, small throw ratio, and short throw
presented unique mechanical and optical challenges. Optically, the
throw distance necessary to even focus the image of a traditional
projection assembly would have necessitated a long arm, further
creating lever amplified stresses on the structure. Even if
structurally sound, a traditional system would have projected a
severely distorted and a relatively small image.
Commercially available electronic front projectors are designed to
project a specified screen diagonal (D) at a specified throw
distance (TD). The throw ratio (TR) of a projector is defined as
the ratio of throw distance to screen diagonal ##EQU1##
Magnification is measured as screen image diagonal/imager diagonal.
Optically, the small throw ratio, off-axis arrangement of the
projection head 106 of projection system 100 requires that the
image simultaneously accommodate three very demanding requirements:
(1) short-throw distance, (2) high magnification, and (3) large
off-axis projection. To minimize image shadowing, in the present
exemplary embodiment, the projector head 106 is located at a
projection angle .apprxeq.15.degree. and the arm measures about 30
in. (.about.76 cms). The screen 102 has a screen (and desired
image) diagonal of 60 inches (.about.152 cm.). Accordingly, the
design goals for the exemplary display system 100 included (1) a
throw distance .ltoreq.800 mm; (2) a magnification.gtoreq.5OX
(.about.69X for 60" diag.); and (3) image distortion correction for
a projection angle .about.15.degree..
The electronic optical engine of the present embodiment includes
imaging and electronic components. As better illustrated in FIG. 7,
in projection system 100 the arm 108 is a rigid hollow structure
surrounded by an outer plastic shell 118. The structure of arm 108
defines an arm chamber 122 and allows for the modular and separate
placement of a control and power electronics module 118 and an
imaging module 120. The control and power electronics module 118
includes control boards, ballast, and other electronic components.
The electronic elements are internally connected through an array
of internal power and data connections. The imaging module 120
includes a light source, projection optics, color wheel and imager.
Those skilled in the art will recognize that a variety of different
modular arrangements may be possible within alternative embodiments
of the present invention. For example, alternatively, components of
the electronics module may be placed inside of frame 104.
Referring to FIG. 7, the projection head 106 includes a lamp unit
132, an imager or light valve 134, condensing optics 136, a color
wheel 138, a condensing mirror 139 and a projection lens 140. The
projection head may also include polarization converters (for
polarization rotating imagers), infrared and ultraviolet absorption
or reflection filters, an alternative light source possibly coupled
with a lamp changing mechanism, reflector mirrors, and other
optical components (not shown). The lamp unit 132 includes a
reflector 131 and a lamp 133.
In the present exemplary embodiment, the imager or light value 134
comprises a XGA digital micromirror device (DMD) having about an 18
mm diagonal, such as those manufactured by Texas Instruments, Inc.,
Dallas, Texas. The imager includes a plurality of pixels, generally
arranged in rows and columns, that form the image. The color wheel
138 is a spinning red/green/blue/white (RGBW) color sequential disc
producing 16.7 million colors in the projected image. In
alternative embodiments, the color wheel and the imager 134 may be
replaced by different suitable configurations, such as a liquid
crystal RGB color sequential shutter and a reflective or
transmissive liquid crystal display (LCD) imager. Those skilled in
the art will readily recognize that other optical components and
arrangements may be possible in accordance with the spirit of the
present invention.
FIG. 9 illustrates a third embodiment of a projection system 300,
where a separate projection engine 306 is placed at a known throw
distance and angle from a screen 302. The projector 306 is tilted
15.degree. from horizontal, but the projection beam is still
30.degree. from horizontal. The throw ratio (TD/D) is equal to or
less than one.
The system 100 allows for the projection head 106 to be placed in
an exact pivotal registration in the operating or projection mode
in relation to the optical screen 102. In system 100, use position
is at a normal arm angle with respect to the screen and generally
above the screen. However, other embodiments may be designed around
other predetermined positions.
FIG. 8 is a perspective view of a second embodiment of a short
throw ratio, offaxis an integrated projector 200 in accordance with
the present invention. An arm 208 holding a head 206 is hinged to a
corner pivot point 210 of a main body 201. When in a use position,
the head 206 is off-axis from an integrated screen 202. Movement
between the two positions may be assisted manually or may be
motor-driven.
The present system offers a novel combination of optical and
electronic means to compensate for the image distortion caused by
the off-axis, small throw ratio (.ltoreq.1) projection. FIG. 10 is
a plan view illustrating distortion components generated in an
uncorrected short-throw, off-axis projection system. This degree of
distortion is a result of the distance from the projector to the
screen being short (throw ratio <1) and the optical system being
off-axis.
There are three main types of distortion components in a small
throw ratio, off-axis system-geometric keystone, geometric
anamorphic and lens-induced pincushion distortion. Each is
explained below.
Traditional throw distance projectors have not had to account for
these types and magnitude of distortion components. As shown in
FIG. 10, the short-throw, off-axis optics can produce a pincushion
distortion component 420, a keystone distortion component 422 and
an anamorphic distortion component 424 which together produced a
combined projected non-rectilinear image distortion 426. The
creation of a rectilinear corrected projected image, in accordance
with the present invention, involves the correction by electronic
and optical means of these three component types of distortion.
Pincushion distortion component 420 is a third order distortion
generated by the wide-angle projection lens, as a consequence of
projecting a high magnification image at a short throw distance.
With respect to distortion component 420, the box 451 represents a
desired undistorted image that may be projected, for example, with
an ideal on-axis projection system. The segment A represents the
undistorted distance from the center of the image to the corner of
the image box 451. The distorted box 421 represents the pincushion
distortion of this ideal image. The segment B represents the
distance from the center of the image to the distorted corner of
distorted image box 421. The pincushion distortion component 420
may be expressed as a distortion percentage defined as:
using the variables shown in FIG. 10. In one embodiment of the
present integrated front projection system, the lens used is a 9.44
mm focal length lens, such as that manufactured by Carl Zeiss Co.
from Jena, Germany, which allows -10.4% distortion, or
approximately 10%, on the design of the lens.
The geometric keystone distortion component 422 occurs because the
optical system is projecting, for example, off axis 15.degree. from
a horizontal plane. In alternative embodiments of the present
invention, the optical system may be off-axis, instead or in
addition, with respect to a vertical plane.
With regard to keystone distortion, FIGS. 11A-11B are cross-section
side elevation views of example optical systems characterized by
being on-axis, and off-axis. As shown in FIG. 11A, an on-axis
optical system, indicated generally at 500, generally comprises a
lamp/reflector 502, an image source 503, an imager 504 and a
projection lens 506. Together, these components of optical system
500 operate to project an image onto a screen 508 along a
projection axis 505. A screen axis 505A extends normal to the
screen center.
Optical system 500 is "on-axis" in that the projection optics
(including the lamp/reflector 502, the center of the imager 504,
and the projection lens) and the projection axis 505 are in
alignment with the center screen axis 505A. As an on-axis system, a
corrected optical system 500 produces an image on screen 508 that
does not suffer from geometric keystone distortion.
In contrast, FIG. 11B shows an optical system, indicated generally
at 610, in which similar projection components are inclined in an
off-axis configuration similar to the integrated front projection
systems described herein. As shown, a projection optics axis 605 is
now not aligned with the center screen axis 605A. In other words,
the image is being projected downward at an angle. Thus, optical
system 600 is referred to as "off-axis" in that the projection
optics and the projection screen are not on a common axis and the
projection lens 606 is positioned away from the screen axis. In
this off-axis case of optical system 610, an uncorrected image 612
projected onto a screen will be distorted (including all three
distortion components), as shown, due to the off-axis alignment.
The distorted projected image would be wider at the bottom than the
top. Similarly, for vertical off-axis projection, the image would
be wider in one side and narrower at the other.
Referring back to FIG. 10, with respect to component 422, the box
453 represents a desired undistorted image that may be projected,
for example, with an ideal on-axis projection system. The distorted
box 423 represents the keystone distortion of this ideal image.
Segment A represents the top width of the image, and segment B
represents the bottom width of the image. Keystone distortion may
be expressed as a percentage defined as:
For example, the calculated keystone distortion for one embodiment
of the present front projection system is 73.9%, or approximately
74%.
Because projection is being performed off-axis, another geometric
distortion component, anamorphic distortion 424 occurs
simultaneously with the keystone distortion 422. As shown by
component 424, anamorphic distortion 424 causes the uncorrected
image to be stretched in the vertical direction relative to the
horizontal direction. With respect to the anamorphic distortion
component 424, the box 455 represents a desired undistorted image
that may be projected, for example, with an ideal on-axis
projection system. The distorted box 425 represents the anamorphic
distortion of this ideal image.
Points A and A' represent the top and bottom edges of desired
undistorted image 455 at the center of the image. Points B and B'
represent the top and bottom edges of the distorted image 425 along
the center of the image, and point B" represent the middle point of
the distorted image.
FIG. 12 is a side elevation view of tilted projection on a vertical
screen that defines the variables used in that equation. The
illustration of FIG. 12 is taken from Rudolph Kingslake, "Optical
System Design," Academic Press, 1983, pp. 269-272, relevant
portions of which are hereby incorporated by reference. Points A.
B. A', B', and B" correspond to the same points in the anamorphic
distortion depiction in FIG. 10.
Point Z represents the "source" point of the projection lens. Point
K represents the intersection point of a horizontal line from point
Z and the vertical plane defined by the screen. The segment from Z
to B" represents the projection axis, where B" is the point at
which the image axis crosses the screen. The screen is represented
by the segment going through points B", B, and B'. The distance "x"
represents the perpendicular distance from the screen at point K to
the point Z, which represents the projection lens (in reality,
complex lens for calculations are modeled as a single lens) on the
projection axis. Segment A to A' represents the image plane that
passes through point B". The angle ".theta." represents the angle
between the image plane and the screen. The distance "x
sec(.theta.)" represents the distance from the point Z to the point
B". The angle ".alpha." represents the angle between the segment Z
to B" and the segment Z to A and the segment Z to A'. The point B
represents the point at which the segment Z to A passes through the
screen. The point B' represents the point at which the segment Z to
A' passes through the screen. As shown in FIG. 12, the segment B to
B" and the segment B' to B" follow the equations below.
The anamorphic distortion percentage may be expressed as:
Anamorphic Distortion [%]=[(x tan(.alpha.-.theta.)+x
tan(.alpha.-.theta.)-2x sec(.theta.)tan (.alpha.))/2x
sec(.theta.)tan(.alpha.)]100
For example, the calculated anamorphic distortion for one
embodiment of the present front projection system is 33.5%, or
approximately 34%.
Referring back to FIG. 10, as shown, the result of distortion
components 420, 422 and 424 is combined image distortion 426. For
this overall distortion 426, the box 457 again represents a desired
undistorted image that may be projected, for example, with an ideal
on-axis projection system. The distorted box 427 represents
combination of the pincushion distortion component 420, the
keystone distortion component 422, and the anamorphic distortion
component 424.
To correct for these distortion components, the present invention
uses a combination of optical and electronic distortion correction.
This combination solution compensates for the distortion components
shown in FIG. 10, and also may compensate for changes in
magnification, alignment and horizontal keystone that may occur in
the process of constructing the projection system.
FIG. 13 is a block diagram of a method for manufacturing a
projection system projecting a distortion corrected image of the
present invention. FIG. 14 illustrates a second embodiment of the
method of the present invention. Rater than rely on purely
electronic methods, with significant data loss, or in purely
optical methods, with significant optical complexity and cost
challenges, the present method details a procedure for implementing
a novel optimized combined optical and electronic solution. The
method illustrated in FIG. 13 establishes one of the boundary
conditions for the maximum limit of electronic correction that may
be combined into the system. In the method illustrated in FIG. 14,
additional steps are used to further determine the optimal
combination of optical and electronic correction means. FIG. 15
illustrates graphically the concept of the method of the present
invention.
Referring to FIG. 13, an initial step in providing a solution is to
assess the projection parameters and expected use for the system.
Projection parameters include the throw ratio, the throw distance,
and the offset angle. An integrated projection system such as those
illustrated in FIGS. 3-7 and 8 offer predetermined consistent
projection parameters. Since the offset angle, screen size and
distance, and desired magnification and focus are constant, the
described integrated systems lend themselves naturally to
optimizing the image in accordance with the present invention.
However, separate systems, such as that illustrated in FIG. 9 also
may be optimized, provided the desired optical parameters are
known. The present method may similarly be applied to rear
projection systems suffering similar data loss and distortion
challenges.
Determining the expected use of the projection system will help
determine the acceptable data loss DL.sub.ACC for the projection
system. For example, constantly refreshed video, where images
rapidly change, may permit higher data loss since the eye does not
have time to examine carefully each frame. In contrast, display of
still data including a variety of details or small print, such as a
database spreadsheet, may not tolerate high data loss.
For example, in the case of the present front projection system, an
acceptable loss of 10% was selected and is represented by line 844
in FIG. 15. This choice was based on the expected use of the system
illustrated in FIG. 8. At this design point 844, the optical design
selection is a micro-imager illumination system with a wide angle
projection lens (9.44 mm focal length diameter lens, 10% pincushion
distortion). FIG. 15 graphically illustrates the concept of finding
an optimal combination of electronic and optical correction of
distortion in accordance with the present invention to accomplish a
rectilinear distortion corrected image. The horizontal axis
represents a spectrum for correcting distortion that can range from
an all optical correction solution to an all electronic correction
solution. On the vertical, one axis represents cost while the other
axis represents loss of image information. Concerning the
combination of optical correction and electronic correction, the
chart of FIG. 15 illustrates two design considerations. Curve 838
shows that the cost and complexity (including factors such as
weight) of the optics for the projection system range from small
for all electronic correction to prohibitively high for all optical
correction (if it is even possible). Curve 840 shows that the image
degradation caused by the use of electronic correction ranges from
small for all optical correction to unacceptably large for all
electronic (again, if possible). According to the present
invention, there is a solution zone, indicated generally at 842,
for how much correction should be handled by the optical components
and how much by electrical components. This design process involves
setting an upper limit on the amount of image information that is
acceptable to lose which reduces the complexity of the optical
design (and thus the cost) until within an area of the acceptable
loss (e.g., 10% residual distortion). Then, an optical solution is
selected at that point, and the remaining correction needed to be
accomplished is done so electronically.
FIG. 17 shows exemplary projector system parameters. The projection
screen is vertical and has dimensions of 922.4 mm high by 1227.2 mm
wide. The projection lens final element is 782 mm from the screen
plane, 74.5 mm above the top of the screen, and the projection lens
axis points downward 15.degree. from the horizontal.
Referring back to the steps of FIG. 13, with the projection
parameters, one selects a first optical design that addressed the
throw ratio, focus, and off-angle constraints. While distortion
components are a factor, they are not taken as the controlling
factor in deciding the optical design at this point.
Having selected an optical design and knowing the projection
parameters allows the calculation of the distortion components for
a not-electronically corrected image under the parameters of the
system. The keystone, anamorphic and pincushion distortion
percentages may be calculated for the system. The distortion
components may be calculated using the exemplary procedures
detailed above or other suitable procedures. In this particular
example, the distortion components for the exemplary system were
calculated to be approximately 10% pincushion distortion, 74%
keystone, and 34% anamorphic.
The shape of the not-electronically corrected or unmodified image
when projected may be derived. A warp map for the correction of the
distortion components. As the exact distance and orientation
between the projector and the screen is known and fixed, a warping
map--tailored to the physical and optical characteristics of the
system--may be prepared. A warp or warping map is defined as the
mapping of the imaging pixels in the imager to yield a shaped or
pre-distorted electronic image on the imager that corrects the not
electronically corrected projected image.
The exact shape of the pre-distorted imager image necessary to
produce a corrected, in this case rectilinear, image on the
projection screen is calculated using a three-dimensional ray trace
that models the optical system as a point at the lens exit pupil,
or alternatively, by an actual ray trace through the system. This
includes a mathematical description of the effects of lens
pincushion distortion, keystone distortion, and anamorphic
distortion.
In FIG. 16, the outline 802 represents an image boundary shaped to
compensate, for example, for keystone and other optical
distortions. Outline 802 also represents a distortion-correcting
image 802 that is shaped to compensate for the calculated amounts
of anamorphic, keystone, and optical distortion. Thus, when
projected, the image would be projected as a corrected rectilinear
image. In particular, the FIG. 16 embodiment of distortion
correcting image 802 would be appropriate for use in an optical
application with a small throw ratio, high tilt angle such as that
described herein. As such, rather than being rectangular, for
example, the nominal (uncorrected) image of this embodiment of
imager 802 has a bottom width of approximately 13.798 mm (6.899
mm+6.899 mm) and a top width of approximately 10.428 mm (5.124
mm+5.124 mm). Also, as shown, the image center correlates to a
position approximately 5.163 mm from the bottom imager 802, and the
overall height of imager 802 is approximately 8.065 mm (2.902
mm+5.163 mm).
The image boundary and the distortion information are mapped and
extrapolated to correspond to the pixels on the imager, creating a
warp map.
Referring to FIGS. 17A and 17B, the locations of the pupil point
PP, intersection point of the optical axis with the screen plane
OX. For this exemplary system PP is defined by the coordinates x=0,
y=-782, z=997.2, and OX is defined by the coordinates x=0, y=0,
z=787.6.
The calculated shape of the pre-distorted image on the imager
accounts for the residual keystone and anamorphic distortion
generated by the 15.degree. downward projection axis, and
.apprxeq.10.4% projection lens pincushion distortion. To calculate
the location of the shaped image edge points, an array of projected
image edge points is defined for the desired projected image size
in the screen plane shown in FIG. 17A. Some examples of points in
the defined 3-dimensional coordinate space would be x=-613.6, y=0,
z=922.4; x=613.6, y=0, z=922.4; x=613.6, y=0, z=0; x=-613.6, y=0,
z=0. These four points define the four corners of the projected
image on the screen plane, with additional edge points defined by
varying x from -613.6 to 613.6 and z from 0 to 922.4 to form an
array of edge points.
A series of line unit vectors from each point on the projected
image edge passing through the pupil point PP is then defined as
EP0. Additionally, a plane is defined which contains the point OX
and is parallel to the plane of the shaped image as in FIG. 17B. If
the same optical system were to project an image on this plane, the
image would have no keystone or anamorphic distortion as the
projection angle would be 0 degrees. In this plane, the
intersection of the projected edge point line unit vectors EP0 is
calculated for each edge point. These points may be described by
the equation ##EQU2##
where PP is the location of the pupil point, L2 is the distance
from point PP to point OX, EP0 is the array of vectors defining the
location of the actual projected image edge points in the screen
plane, and OA is the optical axis unit vector.
Staying in the plane parallel to the shaped image plane and through
the point OX, the effect of optical distortion can be considered by
defining a set of points in this plane that would represent an
image with 0% optical distortion and comparing them to the points
defined by EP1. These points may be described by the equation
where ep2 is the distance from OX to each undistorted point in this
plane, ep1 is the distance from OX to each distorted point in this
plane defined by EP1, and d is a measure of the system distortion,
for this system 0.104 or 10.4%. An array of vectors defining the
locations of each point ep2 may be defined as EP2, and an array of
line unit vectors from the points EP2 through the pupil point PP
can be defined as V2.
A complex optical system may be modeled as a pinhole at the exit
pupil. In such a model, the object and image points are located
along a straight line through the pupil point. Since the effects of
keystone, anamorphic distortion, and optical distortion have been
accounted for in calculating V2, the final shaped image edge point
locations can be defined by calculating each point where a line
from the points EP2 through the pupil point PP intersects the
shaped image plane. These points may be defined in the
3-dimensional coordinate space of this transform as ##EQU3##
Where PP is the pupil point, L1 is the distance from PP to the
shaped image plane along the optical axis, V2 is the array of line
unit vectors from point EP2 through PP, and OA is the optical axis
unit vector. V1 gives the x, y, and z coordinates of the edge
points of the shaped image in the 3-dimensional coordinate space of
this transform.
To plot the outline of the shaped image in the 2-dimensional plane
of the shaped image, the points defined by V1 are translated to the
origin of the 3-dimensional transform space and rotated 15 degrees
about the x axis to give ##EQU4##
where V0 is the distance form the points defined by V1 to the point
where the optical axis intersects the shaped image plane. XZ is
then an array of points in 3-dimensional space which all have y=0,
and the x and z coordinates represent the location of the shaped
image edge point coordinates in the plane of the imager, with the
point x=0, z=0 defined as the point at which the optical axis
intersects the plane. The resultant calculated shape of the
pre-distorted imager image to produce a rectilinear image on the
projection screen, given the expected lens distortion, and the
keystone and anamorphic distortion generated by the off-axis
projection, corresponds to 802 as shown on FIG. 16. For a system
magnification of 88.3x, the shaped image has a bottom width of
approximately 13.8 mm and a top width of approximately 10.4 mm.
Also, the image center correlates to a position approximately 5.2
mm from the center of the lower boundary, and the overall height of
imager 802 is approximately 8.1 mm. The outward curvature of the
image boundary is opposite to the inward curving boundary of a
projected image with pincushion distortion.
The warp map does not merely determine the outer boundary of which
pixels are to be turned on and which pixels are to be turned off.
The warp map also may relate pixels in the original electronic
image to specific new pixel points on the imager. Since the
resolution (number of pixels) of the warped image may not
correspond to the number of pixels in the original electronic
image, the warp map also may determine which pixels of data are not
displayed (i.e., lost pixels).
The concept of image warping has been used for military real time
imaging applications. The image warping functionality of a
microchip, is described for example in the mentioned U.S. Pat. No.
5,594,676, relevant portions of which are incorporated herein by
reference. Real time warping requires extraordinary computing power
and feedback regarding the relative positions and orientations of
the projector and screen.
However, in previous image warping techniques, image information
may be lost through the mapping and extrapolation process. This is
so because, for example, in the top row, an image that is nominally
14.12 mm wide will be compressed into an image that is 10.248 mm
wide. As illustrated in FIG. 16, since the pixel density of a
normal imager is not adjustable, the portion 812 within outline 802
will include fewer pixels than the whole imager size 810, and yet
be trying to convey the same image information. Thus, image
information will be lost. Pixels within portion 814, which is not
used after remapping into portion 812, will simply be dark and
unused to provide image projection. This portion 814, therefore,
represents wasted imager space and lost data. While this data loss
may be considered acceptable in some real time, constantly updated,
displays, it may be unacceptable for certain display
applications.
In the method of the present invention, once the warping map has
been calculated, it is then applied to the electronic imaging
component (i.e., imager or light valve). The warp map is
implemented using an electronic correction component. In the
present exemplary system, the electronic correction component is
selected to be a pre-distortion firmware integrated circuit chip
that electronically pre-distorts the unmodified image according to
the warp map to account for distortion that will occur as the image
passes through the projection system. In a particular embodiment,
the present invention couples image warping technology with the
integrated design of a wall mounted projector. In particular, one
such pre-distortion microchip selected for use in the present front
projection system is a product manufactured by Silicon Video, Inc.
of North York, Ontario, Canada. A warped or pre-distorted image on
the imager results.
The electronic data loss, DL.sub.E of this pre-distorted image in
the electronic imaging component may be measured. One method for
measurement first calculates the entire area of the warped image on
the imager, A.sub.w. The area may be calculated in pixel units or
using other suitable area unit measurements. As the total imager
area A.sub.T is known or easily calculated, a data loss percentage,
DL%, may be calculated, where:
For example, the total number of pixels for an XGA resolution
imager is 1024 horizontal by 768 vertical (786,432 pixels). In one
warped image embodiment the number of pixels actually utilized is
calculated to be 502,680. The electronic data loss DL.sub.E is then
one minus the ratio 502,680/786,432, or about 36%. In this
exemplary projection system, it was determined through image
analysis and user feedback that the acceptable data loss was
approximately 40%.
Other methods calculate the maximum data loss at a specific row or
column on the warped image. The width, in pixels, of the narrowest
row of pixels in the pre-distorted image is measured. Similarly,
the height of the shortest column of pixels may be used. The result
is then divided over the known number of available pixels in that
row or column. These calculations yield maximum vertical and
horizontal data loss.
The next step is to compare the electronic data loss with the
acceptable data loss for the system. If DL.sub.E.ltoreq.DL.sub.ACC,
that is, the data loss is acceptable, the system is within the
acceptable design parameters.
If DL.sub.E >DL.sub.ACC, then the next step is to provide an
optical correction mechanism that reduces the distortion
components. The above steps of calculating the distortion, deriving
and applying a warp map, and measuring the data loss are repeated
until DL.sub.E <DL.sub.ACC. In the present exemplary embodiment,
electronic correction alone yielded data loss in excess of the
acceptable 10%. To provide keystone distortion correction, the
light valve center was shifted from the projection lens center to
correct a portion of the keystone distortion. Different correction
techniques may be combined. For example, keystone correction also
may include screen inclination. In an alternative embodiment, the
screen may be motor driven, to reach an inclined projection
position at the time that the arm is placed in the open
position.
FIG. 18 illustrates an off-axis optical system in accordance with
the present invention, indicated generally at 900, with optical
correction for geometric keystone distortion. As illustrated in
FIG. 18, the projection optical axis is displaced 3.7 mm from the
optical center of the imager for optical correction of .apprxeq.58%
of the total keystone distortion.
However, full keystone correction by off-setting the imager center
from the projection axis presents difficulties in extreme off axis
systems. As the magnitude of offset increases, the off-setting
technique requires that the imager be farther off-center. A lens
design has a determined object field, or simply field of the lens.
At some point, the imager falls out of the object field of the lens
design. While, in some cases, it may be possible to redesign the
lens to increase the field, such a redesign usually involves added
optical costs, size, weight and/or complexity. Furthermore, as the
imager is offset and the lens design changes, pincushion distortion
is affected.
In the present exemplary analysis, the projection lens pincushion
optical distortion is corrected electronically by the warp
transformations. The off-axis geometric keystone and anamorphic
distortions are partially corrected optically by transverse
displacement of the projection lens relative to the imager, and
partially corrected electronically by the warp transformations. In
a completely uncorrected off-axis system there is an allowed 10%
pincushion lens distortion, resultant 74% keystone distortion and
34% anamorphic distortion. By optical and electronic means, these
distortions are reduced to a negligible amount. The proportional
mix of these corrections is given in the following table.
Total Proportion of Proportion of Distortion Uncorrected Optical
Electronic Final Corrected Type Distortion Correction Correction
Distortion Pincushion 10% 0.0 1.0 0% Keystone 74% 0.58 0.42 0%
Anamorphic 34% 0.76 0.24 0%
FIG. 14 illustrates a further refinement to the method illustrated
generally in FIG. 13. While the initial steps are the similar,
after it is determined that DL.sub.E <DL.sub.ACC, the question
is asked on whether the cost targets and system constraints are
met. System constraints may include factors such as size, weight,
safety, regulatory compliance, manufacturing complexity,
reliability or other facts considered important for the specific
optical design. If these requirements are not met, the method of
the present invention returns to the step of selecting the optical
design. This additional decision step allows the designer to
analyze whether the optical design has been overengineered and
whether the optimal combination of electronic and optical
correction has been selected. Conceptually, the first decision
point, DL.sub.E.gtoreq.DL.sub.ACC, and the iterative process it
regulates determine the right boundary of the optimal solution zone
842. The second decision point and its iterative process help
determine the left boundary of zone 842.
In accordance with the present invention, a combined electrical and
optical solution is provided for correcting a projection image from
non-linear distortion (including the effects of keystoning) as that
distortion is exemplified in FIG. 10. This highly coupled system
can comprise a projection illumination system that encompasses a
multiple element projection lens (with a focal length of lesser or
equal to the imager height), and approximate f-number of f/2.8 in
conjunction with the electronic front end firmware. The projection
lens is designed to handle a wide angle/short throw optics by
optimizing the relationship between field angle, focus uniformity
and illumination uniformity. Because of cost and size
considerations for the projection lens, it is desirable to retain
some optical distortion; e.g., a pincushion distortion in the lens
design likewise, less than full correction of keystone distortion
may be desired. As described above, the electronic part of the
solution consists of the firmware chip which utilizes a correction
technique that pre-distorts the output image to compensate for the
distortions that will be introduced by the rest of the system. The
end result is a corrected screen image. It should be understood
that effective electronic correction requires an accurate and
precise description of the distortion and a means of mapping the
original input data to the properly corrected output.
Those skilled in the art will readily appreciate that elements of
the present invention may be combined, separately or in one system,
to provide videoconferencing, data-conferencing, and electronic
whiteboard functions, as well a any other function where a light
and compact display system may be useful.
As the system of the present invention is designed to optimize the
projection image at the predetermined projection position, no
set-up adjustments are necessary to the optics, mechanics, or
electronics and optimal on-screen performance is consistently
offered. The integral structure of the system allows for easier
storage and portability and avoids cabling and positioning
associated with the use of traditional projectors.
Those skilled in the art will appreciate that the present invention
may be used with a variety of different optical components. While
the present invention has been described with a reference to
exemplary preferred embodiments, the invention may be embodied in
other specific forms without departing from the spirit of the
invention. Accordingly, it should be understood that the
embodiments described and illustrated herein are only exemplary and
should not be considered as limiting the scope of the present
invention. Other variations and modifications may be made in
accordance with the spirit and scope of the present invention.
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